Resilient CRS-2 Dragon successfully berthed on the ISS

March 2, 2013 by Chris Bergin

Back to full health, SpaceX’s Dragon spacecraft was captured and then berthed by the Canadarm 2 on the International Space Station (ISS) Sunday morning. Arriving just one day later than planned – due to required troubleshooting with the spacecraft’s propulsion system – Dragon marked its third arrival at the orbital outpost.

Only one minor issue was reported prior to launch, namely a low temperature reading on the flight computer – caused by a combination of chilly conditions in the region and the “cold soak” from the LOX tank on the Second Stage during the countdown.

The Falcon 9’s ascent was nominal, with just one reference to a Ground Support Equipment (GSE) issue after lift-off, relating to a small fire on the pad – as is usually seen when the Falcon 9’s plume catches the pad’s umbilical line on the strongback.

“Post-separation, Dragon’s propellant system opened valves between the Helium tanks and the Fuel and Oxidizer tanks to pressurize the system. All four fuel tanks were observed to pressurize as expected. However, only oxidizer tank 1 showed nominal pressure response,” the notes added.

With three of the four quads at a lower than required pressure, only “Quad 1” remained online, which was not enough to ensure Dragon could maintain attitude control.

“Ox tanks 2,3, and 4 did not indicate a pressure increase. Due to this failure to pressurize, three of Dragon’s four prop quads were unusable. Dragon’s attitude drifted while troubleshooting occurred, and solar array deploy was delayed.”

Although the initial plan was to deploy Dragon’s solar arrays once two Quads were available, SpaceX decided to unfurl the arrays with only one Quad working. This proved to be helpful for the unstable Dragon, with SpaceX CEO Elon Musk describing it as not unlike an ice skater stretching out her arms during a spin maneuver to slow the rotation.

Meanwhile, controllers continued to evaluate the propulsion system, concluding it was was being caused by either a blockage in the helium line, or a sticky check valve. The solution was to “jack-hammer” the valves in the system by commanding them to cycle on and off several times in succession. This proved to be successful.

“Troubleshooting involved cycling the helium isolation valves between the He and Ox tanks 2-4. This allowed “slugs” of pressure to flow through the regulators and downstream to in-series check valves that were suspected to be the cause of the blockage,” added the notes. “After several cycles, the Quad 4 Ox tank suddenly rose to its expected pressure, indicating the blockage had cleared.”

With two Quads (1 and 4) now available, Dragon regained attitude control. Shortly after, the remaining two Quads (2 and 3) saw their tank pressures eventually return to normal.

Now with all four Quads, a much happier Dragon conducted a test burn of around 30 seconds to observe the performance of its prop system.

“All appeared well, as oxidizer tank pressures remained stable and did not decay, as might occur with restricted helium flow,” the notes continued. “This was followed shortly by a four minute burn, during which the prop system again appeared to function nominally.”

Notably, the delay rescheduled the key Coelliptic Burn, resulting in the Dragon being unable to arrive at the ISS on Saturday. However, thanks to the speedy mitigation of the problem by the SpaceX team – and what appears to be good propellant margins – Dragon successfully negotiated a path to be in a stance for a Sunday berthing.

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“Several subsequent burns have been performed – including the Height Adjust Burn and CE-1 burn – and the prop system continues to appear to function well, though SpaceX is evaluating post-burn data to confirm.”

With NASA concurring with SpaceX’s healthy status for Dragon, Sunday morning saw the triumphant spacecraft arriving 2.5 km below ISS, ahead of a Go/No-Go for the HA3/CE2 burn pair, resulting in Dragon closing to just 1.2 km distance from its destination.

The HA3/CE3 burn pair, using RGPS and configured with the ISS’ own GPS system, were then conducted, followed by the HA4 (Ai) burn, taking Dragon inside the corridor where the crew began to monitor the spacecraft’s approach.

When all parties are satisfied with Dragon’s performance – and ability to abort if required – Dragon was then given a “Go” to approach to 30 meters distance from the Station where it automatically paused.

At all points, the ability to abort could be made by either controllers on the ground, the Dragon itself or the ISS crew – via the Commercial Orbital Transportation Services (COTS) Ultra High Frequency (UHF) Communication Unit, or CUCU, which rode in the middeck stowage locker on Atlantis during STS-129 late in 2009, before being handed over to ISS crewmembers ahead of the demonstration flights.

Upon receiving the “Go for Capture” call from Houston, the ISS crew armed the SSRMS capture command and begin tracking the vehicle through the camera on the Latching End Effector (LEE) of the SSRMS. NASA Expedition 34 Commander Kevin Ford and NASA Flight Engineer Tom Marshburn are the crewmembers then completed the task of capturing the CRS-2 Dragon.

With the ISS’ thrusters inhibited and Dragon confirmed to be in free drift, the arm’s LEE closed over the Grapple Fixture (GF) pin on Dragon to trigger the capture sequence ahead of pre-berthing maneuvers.

The Dragon, secured by the SSRMS, was then carefully translated to the pre-install set-up position, 3.5 meters away from the Station’s module, allowing the crew to take camcorder and camera footage of the vehicle through the Node 2 windows.

The SSRMS then translated Dragon to the second pre-install position, at a distance of 1.5 meters out. Desats were inhibited prior to the maneuver of the Dragon into Common Berthing Module (CBM) interface to begin the securing of the spacecraft to the ISS.

A “Go” at this point was marked by all four Ready To Latch (RTL) indicators providing confirmation on the RWS panel.

Dragon was eased through first stage capture tasks, allowing the SSRMS to go limp, ahead of second stage capture, officially marking Dragon’s berthing with the ISS.

The hatch between the newly arrived SpaceX Dragon spacecraft and the Harmony module was opened at 1:14 p.m. EST.